A battery can deliver current to a load such as an engine, a light, or a radio for a considerable length of time. As long as the potential difference between its terminals remains close to a nominal value and a sufficient current is generated, current will flow to the load. As the current flows, the battery's capacity to drive the load becomes diminished and may eventually become exhausted.
To replenish this loss, a battery may be connected to a source that provides both positive and negative charges to its electrodes. It is these charges that establish the potential difference and the current needed to drive the load. The charges are transported from one terminal to the other in a direction that is opposite to the bias of the terminals. One device that accomplishes this transport is an automobile battery charger.
A known automobile battery charger converts 120-volts, 60-Hz AC-line, to a 12-volt DC output through a power transformer, a bridge rectifier, and a filter. The power transformer consists of two separate windings that are magnetically coupled to each other. The first winding is connected to the 120-volt AC-line, and the second winding is connected to the bridge rectifier. Through a selection of an appropriate turn ratio, the AC-line voltage is stepped down to a 12-volts AC-line output. The bridge rectifier converts this stepped down AC-line output to a rectified output that is not always suitable for charging batteries. By eliminating the time varying magnitudes of the rectified output through a filter, the output of the battery charger is stabilized and is suitable for charging a battery.
Conceptually, the described battery charger is relatively simple and can be made inexpensively. The large size of the power transformer needed in many battery chargers can make the battery chargers very heavy. Other battery chargers may not provide a reliable charge or a reliable charge termination. Overcharging a battery can reduce a battery's life and lead to the emission of dangerous corrosives. If battery terminals are insulated by sulfates, some battery chargers will not deliver sufficient current to charge the battery. In this condition, the battery becomes an open circuit.